/writing-tests — How tests are written in this codebase

The goal is tests that survive a full rewrite of the implementation, as long as the behavior stays the same. If your test breaks when a private method is renamed or a call count changes, you're testing the wrong thing.

The rule

Assert on observable state, not on mock calls.

Drive the code through its user-facing surface (editor.copy(), editor.click(), toolManager.handleKeyDown(...)) and assert on what a user would see — glyph contours, selection, viewport pan, tool state, command history, returned values. Never on "was method X called N times."

Everything else here is a consequence of that rule.

When not to add a test

Do not treat line coverage or "every changed file needs a test" as the goal. Add a test only when it protects an observable behavior, domain invariant, or public boundary that could regress independently of the implementation.

Skip a new test when it would only:

• repeat declarative framework wiring, dependency setup, or static configuration;
• snapshot private query keys, option objects, CSS classes, component structure, or other implementation choices;
• exercise a trivial delegation whose behavior is already covered at the owning boundary;
• justify extracting a tiny helper solely so the implementation has something easy to unit-test;
• assert behavior already guaranteed by TypeScript, Rust's type system, or the library being configured;
• require mocks, spies, a parallel harness, or broad DOM/IPC infrastructure for code that is better verified by focused manual QA.

Examples: do not test that a QueryClientProvider is mounted, that a query key contains a particular string, or that a fixed-size chunk helper calls slice() correctly. Do test stable batching if it is exposed as a reusable contract with edge cases, or test the user-visible result through a real integration boundary when one exists.

When no worthwhile automated test exists, say so explicitly in the handoff and record the focused manual verification performed. Do not create a low-value test to make the diff look complete.

Deleting or replacing tests

Tests follow behavioral ownership, not implementation names. Removing or replacing a class does not make its tests obsolete when another class now owns the same behavior.

Before deleting a test or materially reducing a test file, create a test migration ledger. For every removed it() or test() block, record:

1. The observable truth or domain invariant it protects.
2. The surviving or replacement test that protects that truth.
3. If there is no replacement, the intentionally removed product behavior that made the invariant obsolete.

Move replacement tests to the new behavioral owner before deleting the old file. Similar-looking coverage is not enough: verify that it exercises the same boundary, failure mode, and state transition. Report removed and replacement test counts in the handoff.

Example:

Removed test	Protected invariant	Replacement
Draft cancel restores rule-expanded points	Cancellation restores every previewed position, including positions outside the initial targets	GlyphLayerEdit.test.ts multi-position cancellation test
Next draft starts from committed preview	A finished edit becomes the base of the next interaction	GlyphLayerEdit.test.ts sequential edit test

If you're stuck, research — don't invent a mock

The moment you think "I'll just mock this out" is the moment to stop and look for prior art. Real Electron / font-editor / reactive-signal codebases have solved the same class of problem: VSCode, Obsidian, Signal Desktop, Bitwarden, Fontra, tldraw. Their patterns are on GitHub.

The SystemClipboard adapter in this repo came from a short WebSearch pass through VSCode's IClipboardService + TestClipboardService — not from inventing one locally. Ten minutes of searching usually beats an hour of mock wrangling, and the resulting test survives refactors because it mirrors a pattern that works in production code.

Rule of thumb: if the repo doesn't already show you a clean way to test this, the answer is probably "inject a boundary and test through it." Go find how someone else injected the boundary before reaching for vi.mock.

When to stop and rethink

If you catch yourself doing any of the following, stop. You're about to write a test this codebase has deliberately deleted.

Reaching for…	What it means	Do instead
Vitest mock primitives — vi.fn, vi.spyOn, toHaveBeenCalled, mock.calls.length, .mock.calls, execution-order arrays (applied.push(...))	Asserting on "was this called" instead of "what did the user see"	Find the user-facing consequence (state change, return value, emitted payload). If there isn't one, the behavior isn't worth a unit test. Exception: when you're testing a primitive whose contract is the invocation count (reactive library fire counts, pub/sub dispatch), a plain closure counter (let n = 0; ...; n++) is fine — don't reach for vitest mocks
Hand-built events, states, or snapshots — constructing ToolEvents, Coordinates, GlyphSnapshots inline so you can pass them straight into a method	Recreating the pipeline instead of using it	Drive through TestEditor — real gestures, real events, real state
Global stubs — vi.stubGlobal, monkey-patching window, requestAnimationFrame, electronAPI	The code has an unmanaged global dependency; stubbing papers over it	Inject the boundary. For rAF: toolManager.flushPointerMoves(). For IPC: SystemClipboard-style adapter. If the boundary doesn't exist yet, add one
Parallel-world test harnesses — mock engines, mock renderers, mock command contexts, raw new Editor(...) in a .test.ts	Reimplementing production in TypeScript so the test can "look inside." Drifts from real behavior; the mock/prod divergence is the whole pain the repo swept out	Use TestEditor. If you can't, extract the logic into a pure function and test that. new Editor(...) in tests is enforced against by oxlint no-raw-editor-in-tests

Test categories

Category	Use when	Template in repo
Tool test	User-triggerable behavior of a tool (pointer, keyboard, selection, cursor)	lib/tools/pen/Pen.test.ts, lib/tools/hand/Hand.test.ts, lib/tools/shape/Shape.test.ts
Command test	A Command's execute/undo/redo round-trip	lib/commands/primitives/PointCommands.test.ts
Pure module test	Stateless class or function with no Editor dependency	lib/tools/text/TextRunController.test.ts, lib/editor/hit/boundingBox.test.ts
Bridge test	NativeBridge against the real Rust engine	bridge/NativeBridge.test.ts

If your target doesn't fit one of these, stop and ask — don't invent a new shape.

Decision: which one

1. Can a user trigger it with a click, drag, or key? → Tool test via TestEditor.
2. Is it a single command's undo/redo contract? → Command test.
3. Is it a pure function or pure class with no Editor? → Pure module test.
4. Is it the NAPI boundary? → Bridge test.

Templates

Tool test

import { describe, it, expect, beforeEach } from "vitest";
import { TestEditor } from "@/testing/TestEditor";

describe("Hand tool", () => {
  let editor: TestEditor;

  beforeEach(() => {
    editor = new TestEditor();
    editor.startSession();
    editor.selectTool("hand");
  });

  it("drag pans the viewport by the screen delta", () => {
    const startPan = editor.pan;

    editor.pointerDown(0, 0);
    editor.pointerMove(50, 30); // crosses drag threshold
    editor.pointerMove(120, 80);
    editor.pointerUp(120, 80);

    expect(editor.pan.x).toBe(startPan.x + 120);
    expect(editor.pan.y).toBe(startPan.y + 80);
  });
});

Pure module test

import { describe, it, expect } from "vitest";
import { SnapPipelineRunner } from "./SnapPipelineRunner";

describe("SnapPipelineRunner", () => {
  const runner = new SnapPipelineRunner();

  it("point-to-point wins over a closer metrics candidate", () => {
    const result = runner.runPointPipeline(
      [
        pointStep("p2p", hit("pointToPoint", { x: 110, y: 110 })),
        pointStep("metrics", hit("metrics", { x: 101, y: 101 })),
      ],
      pointArgs,
    );

    expect(result.source).toBe("pointToPoint");
    expect(result.point).toEqual({ x: 110, y: 110 });
  });
});

Stub inputs (a PointSnapStep here is an interface — build a minimal one). Never stub the class under test.

Bridge test

import { describe, it, expect, beforeEach } from "vitest";
import { NativeBridge } from "./NativeBridge";
import { createBridge } from "@/testing/engine";

describe("NativeBridge session lifecycle", () => {
  let bridge: NativeBridge;

  beforeEach(() => {
    bridge = createBridge(); // real Rust engine via shift-node
  });

  it("startEditSession populates $glyph", () => {
    bridge.startEditSession("A");
    expect(bridge.hasSession()).toBe(true);
    expect(bridge.$glyph.peek()).not.toBe(null);
  });
});

The fake-test checklist

Before committing a test, run through these. Any miss means the test is wrong.

1. Mental deletion. Replace the method body under test with throw new Error("unimplemented"). Does your test fail? If it still passes, you wrote a tautology — usually because you asserted on a value you constructed rather than on a consequence.
2. User-facing surface. You drove through a method a real user can trigger (pointer, keyboard, command, menu action), not a #private field or a tool-internal method.
3. Specific assertions. expect(editor.pointCount).toBe(4) — yes. expect(result).toBeDefined() or expect(spy).toHaveBeenCalled() — no, those survive any implementation.
4. Correct code path. If the real production path goes through command history + bridge + signals, your test goes through those too. You didn't reach behind the facade.
5. Under ~15 lines. If a single it() body exceeds ~15 lines, it's testing too much at once, or its setup belongs in beforeEach.

Setup discipline

• beforeEach is ≤ 5 lines: new TestEditor() + startSession() + selectTool() + optional fixture.
• No wrapper factories around TestEditor. If you need a reusable helper, it belongs as a method on TestEditor itself (see pointerMove, click, escape).
• If your test needs a pre-drawn shape, draw it with the pen/shape tool in beforeEach — don't construct glyph snapshots inline.

Naming

• describe() names should explain the behavior or contract under test, not just repeat a class or file name.
• Prefer describe("glyph source geometry follows coordinate patches", ...) over describe("GlyphSourceState", ...).
• A class name can appear inside a larger phrase when it adds clarity, but the phrase should still tell the reader what invariant is being protected.

When testing is genuinely hard

Some code resists clean unit testing — DOM event handlers, focus management, IME composition, React effect lifecycle.

Default move: extract the non-DOM logic into a pure function and test that. Example: HiddenTextInput keyboard handling → lib/tools/text/textInput.ts → handleTextKeyDown(event, editor). The component becomes a thin adapter; the extracted function is a normal tool test via TestEditor. The part that genuinely can't be tested cleanly shrinks to ~20 lines that rely on manual QA.

If after extraction and research you're still reaching for jsdom + @testing-library/react + IPC mocks, pause and ask the user before introducing that infrastructure.

Why these rules exist

The codebase went through a deliberate sweep that deleted thousands of lines of mock-based tests. See commits 5f2f503, 876e542, 642ec99, bfea0b5, 3f3a6d6, fa8a829, bd07e9d, 3f3a6d6 Delete MockFontEngine. The motivating pain was real:

• Mocks drifted from the real Rust behavior they pretended to simulate. Tests passed; production broke.
• Spy-count tests locked in implementation details; every refactor broke tests that weren't guarding any behavior.
• Global stubs (vi.stubGlobal("window", ...)) leaked across test files and hid unmanaged global dependencies.
• Mock context builders (services.ts, 1,795 lines) became their own maintenance burden.

The replacement — real Editor, real Rust via NAPI, fake only at the outermost boundary (SystemClipboard, NativeBridge) — catches regressions mocks silently missed. Don't reintroduce what was deleted.

Visual E2E reliability

Before adding or changing screenshot assertions, follow Desktop E2E capture determinism.

• Control host-dependent capture inputs, including native scrollbar gutters; fixed viewport dimensions alone are insufficient.
• Verify the installed capture API and use identical normalization for golden assertions and review attachments. Preserve behavioral and visibility checks on the unmodified layout.
• Inspect baseline differences, then verify without snapshot updates or retries. Repeat under the environmental condition that caused the failure, not only the environment that already passed.
• Do not use increased tolerance, retries, or replacement baselines as a substitute for explaining a mismatch. Skipped E2E checks do not establish merge readiness.

Running

pnpm test              # full vitest suite, runs against real Rust
pnpm test:watch        # watch mode
pnpm typecheck         # tsgo across all packages
pnpm lint:check        # oxlint, includes no-raw-editor-in-tests and no-mock-call-assertions

All three must pass before committing. If no-mock-call-assertions flags your test, it's caught you reaching for the banned pattern — fix the test, don't disable the rule.